1. Field of the Invention
The present invention relates to patterning processes using materials such as a conductive metal paste and a solder flux in the fabrication of an article such as an electronic component and, more particularly, to screening or masking processes for applying coatings of materials for forming patterns on layers of a multilayer ceramic (MLC) circuit structure whereby the coated material and/or electronic component may be identified to verify if the proper coating material and/or the proper electronic component was employed.
2. Description of Related Art
Forming patterns of a material on a surface by extruding or forcing the material through, for example, a screen or mask, such as a perforated sheet, has been applied in many fields such as printing and engraving, graphic arts and manufacturing processes, particularly in the electronic industry. The ability to form a plurality of conductors simultaneously on the surface of a substrate permits an inexpensive alternative wiring technique which also results in a compact efficient wiring structure. The terms mask and screen will be used interchangeably herein to indicate coating procedures.
The scale of integration in integrated circuits has steadily increased over the years, resulting in integrated circuits with extreme complexity and capable of very comprehensive functions. A particularly successful type of structure with such devices has been developed which uses a plurality of layers of glass, ceramic or other insulative material with conductors formed on the respective surfaces and in through-holes (vias) in the layers. These components are typically termed multilayer ceramics (MLC) and since circuits constructed in this way are three dimensional, a high degree of complexity is possible.
In such structures, the conductors are usually formed by applying the desired pattern on the substrate using a conductive paste. The conductive paste pattern is usually formed by extruding the paste, which is usually highly viscous, through a stencil or mask such as a perforated sheet by passing an extrusion nozzle over the mask located on the ceramic layer or greensheet of the MLC.
In general, a multilayer ceramic semiconductor package (MLC) is formed by stacking and bonding together flexible paper-like sheets commonly referred to as ceramic greensheets. Greensheet segments of desired size and configuration are punched to provide via holes and, by a screen printing technique, a conductive paste fills the via holes and/or a conductive circuit pattern is applied to the face of the greensheet as required. Such patterned greensheets, after screening, are assembled in a stack, pressed and subsequently sintered in an oven at a relatively high temperature. Upon sintering, the solvent, vehicle, and binder material used to form the greensheet are burned off providing a rigid unitary ceramic body having interior interconnected conductive patterns. Critical to the manufacturing process is the screening operation since the ceramic greensheets are relatively fragile, their thickness generally on the order of 1 to 20 mil and their surface area is relatively large compared to their thickness.
There are many types of MLC conductive pastes which are screened onto the greensheet. An electronic conductive molybdenum paste is disclosed in U.S. Pat. No. 4,576,735, which patent is hereby incorporated by reference. In general, a metallic paste such as a molybdenum paste comprises molybdenum power, a solvent and a binder. A plasticizer may also be employed to control viscosity and pattern formation. The concentration of the various ingredients may vary widely with the proviso that a paste is formed which has a proper viscosity and which may be effectively applied to the greensheet or other surfaces by screening.
Typical solvents for the metal paste include n-butyl carbitol acetate, a terpineol, n-butyl carbinol and the like. The binder includes materials such as ethyl cellulose, polyvinylbutyral and the like. The metal material has a fine particle size generally less than 10 microns on average. Typically, the metal pastes are prepared by adding the ingredients to a mill and mixing the ingredients until a homogenous paste is obtained.
In general, a complete patterning process for greensheets may be described as follows:
A ceramic slurry is prepared by mixing alumina (ceramic) powder, organic binder, plasticizer and solvent at a given mixing ratio in a conventional manner. The slurry is shaped into a tape by means of a tape caster and thereafter the resulting tape is dried and cut into a given length to form a plurality of unsintered soft ceramic sheets or so-called "greensheets".
Via holes are then punched in the sheets and circuit patterns are printed on the surface of each greensheet and in the vias with a metallizing ink or paste using a screen or mask printing process. The metallizing paste, as noted above, is typically a molybdenum paste although other metallic pastes such as tungsten may also be used. The sheets are then dried, stacked, laminated and thereafter sintered to form the MLC substrate.
While the above description was directed to a metallic conductive paste to form conductive lines and to fill vias in greensheets for MLC fabrication, it will be understood by those skilled in the art that there are a number of other coating materials used in the fabrication of MLC electronic components and other electronic components such as printed circuit boards, heatsinks and covers. Exemplary of such coating materials are solder fluxes, solder, photoimageable inks, solder resists, adhesives, lubricants and thermal heat transfer compounds.
The fabrication of electronic components requires a large number of steps as well as the use of a large number of coating materials such as the conductive paste, solder fluxes, etc. mentioned above. Even within a particular class of coating material such as a molybdenum conductive paste, different pastes may be used depending on the properties required for the paste such as density, viscosity, electrical conductivity, etc. If the incorrect printing (coating) material is used and/or an incorrect substrate is used, the integrity of the final electronic component is compromised and the component will most likely be rejected.
With regard to other coating materials used in screening processes, solder and/or solder fluxes may be applied through a mask to a sintered substrate in preparation for a solder bonding operation such as a C-4 operation. Photoimageable inks may also be applied by a screening method to form a pattern on a substrate which can then be developed and metallized to form the desired circuit pattern. Additionally, a solder resist material can be applied to the surface of a substrate through a screening operation to protect the substrate from the high temperatures encountered during a solder reflow process.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a coating material such as a conductor paste, solder flux, or other such material which material is used in the fabrication of electronic components and which material contains an identifying component which can be identified (measured) to identify the coating material.
A further object of the present invention is to provide a method of identifying a coating material which may be used in any part of the electronic component fabrication process to identify and verify that the proper material was used in the process and, preferably, that it was used on the proper electronic component substrate which is also identifiable.
An additional object of the present invention is to provide an electronic component made using a metallized conductive paste or other coating material of the invention which material is applied to a substrate of an electronic component and which material contains an identifying component which is identifiable so that the integrity of the electronic component can be verified.
It is yet another object of the present invention to provide a process for fabricating an electronic component with at least one of the steps being the use of a coating material which is applied to a substrate of the component such as a greensheet and which material contains an identifying component which can be identified to verify the use of the proper coating material on the proper substrate and, consequently, verify the integrity of the electronic component.
Other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.